1. Field of the Invention
The present invention relates to sub-port multicasting in an Asynchronous Transfer Mode (ATM) switching system and, in particular, to multicasting by extracting the sub-port fan-out information inserted in the local header of cells.
2. Background of the Related Art
Generally, the ATM method divides user information into packets of a particular size, inserts destination information into header parts of packets, transfers packets in the form of cells of a fixed size, and restores original information from communicated packets. The ATM switching system provides a header address value, among the destination information, to a user terminal. The header address value is attached to the front of the user information, before transmission. Thus, the switching system may easily recognize the user information, convert it into cell unit packets, and then transfer them to the next destination.
The overall structure of such an ATM switching system is illustrated in FIG. 1. The ATM switching system is composed of a plurality of input modules (11) that process the input of user cells, a plurality of output modules (14) that process the output of user cells, a control module (13) that controls signal processing or call processing, etc., and a switch module (12) that performs switching into a relevant output module (14), by selecting the output path of cells according to path selection information attached to the front of the cell headers inputted from input module (11).
Input modules (11) and output modules (14) are subscriber boards, which physically access the subscriber lines.
Switch module (12) connects input modules (11) and output modules (14), which are the subscriber boards, through a plurality of switch fabric ports (hereinafter, “switch port”) and performs switching between input modules (11) and output modules (14). According to the relevant related art, if a plurality of sub-ports with narrow bandwidths exist in one switch port, multicasting for the sub-ports is implemented by maintaining a sub-port multicast table and referring to the table using the connection ID. The connection ID is reserved at the local header of a cell that is transmitted transparently through the switch fabric.
As shown in FIG. 2, the related art sub-port multicasting of an ATM switching system operates as follows. The illustrated ATM switch system includes egress port cards (40) and ingress port cards (20), each having four sub-ports. Each sub-port is implemented with a 1 giga port device at the switch port of a switch fabric (30), which has 4 giga interfaces. In the related art sub-port multicasting method, a unique connection identifier is reserved in advance, at system level, and used for the purpose of the sub-port multicasting.
The multicasting for switch ports of the switch fabric (30) is performed by assigning a connection ID, for the ATM connection, to each switch port at the ingress port card (20) of the ATM switch system. Additionally, a virtual connection parameter table is stored and maintained in memory that has the connection IDs and fan-out port information, so that the multicast fan-out port information may be referred to through a connection ID. That is, multicasting is performed using the fan-out port information inserted into the local headers of multicast cells.
The structure of the multicast cell is illustrated in FIG. 3. In FIG. 3, the fan-out switch port information is the information in the ‘PortBitMap’ field and the connection ID is the information in the ‘Connection ID[15:8][7:0]’ field.
The virtual connection parameter table for switch-port multicasting, as shown in FIG. 4, includes the Connection ID field, which is assigned to each switch port for ATM Virtual Channel (VC) connection, and the PBM field (PBM [7:0]˜PBM [31:24]), which sets forth fan-out switch port information. The fan-out switch port information indicates the egress port that is to perform the switch-port multicasting, using the connection ID in the bitmap format. If the connection ID indicates a unicast cell, 1-bit fan-out switch port information for the unicast cell output is set at the PBM field. If the connection ID indicates a multicast cell, fan-out switch port information of 1 or more bits is set at the PBM field.
Also, for multicasting to sub-ports, the egress port card (40) shown in FIG. 2 must store and maintain in memory the sub-port multicast table (41), which has the same number of entries as the number of connection IDs that are reserved, as illustrated in FIG. 5. If a cell transmitted transparently to the switch fabric (30) is a multicast cell, the ATM switching system refers to the sub-port multicast table (41), using the connection ID included in the local header of the relevant cell. Then, the ATM switching system copies the cell for each of the fan-out sub-ports, according to the result of the reference to the table, and outputs the copied cells to relevant subscribers through a multiplexer (42).
In other words, in a sub-port multicasting ATM switching system of the related art, not only is a virtual connection parameter table used for switch port multicasting at the side of the ingress port card (20), but also a separate sub-port multicast table (41) is used at the egress port card (40). For each switch port, a sub-port multicast table (41) is maintained by reserving in advance a restricted number of connection IDs, for the purpose of the multicast connection.
For the sub-port multicasting operation in an ATM switching system of the related art, the ingress port card (20) assigns connection IDs for the pre-reserved multicast area, at the time the multicast connection is set-up, and then sets the virtual connection parameter table and the sub-port multicast table (41) for each fan-out switch port in accordance with the relevant connection ID.
Thereafter, when a cell is inputted into the port device where a virtual connection parameter table is located, the connection ID is retrieved using the value of the Virtual Path Identifier (VPI)/Virtual Channel Identifier (VCI) and the Multiple Physical Device (MPHY) information inserted in the header of the relevant cell. Then, the cell is checked to determine whether it is a multicast cell.
If it is determined that the cell is a multicast cell (in other words, if a cell having an associated connection ID assigned for multicasting is inputted), the fan-out switch port information is recorded in the bitmap format at the local header of the relevant cell and the multicast indicating field is set to indicate that the relevant cell is a multicast cell. Then, the cell is transmitted to the egress port card (40) through the switch fabric (30).
Thereupon, the egress port card (40) extracts the connection IDs and the multicast indicating field from the local header of the cell transmitted through the switch fabric (30). Then, the egress port card (40) obtains the fan-out sub-port information for multicasting, by referring to the sub-port multicast table (41), using the extracted connection ID as an index. Then, cells are copied and outputted for the relevant fan-out sub-ports using the fan-out sub-port information extracted from the relevant sub-port multicast table (41).
Recently, as the amount of high-quantity multimedia information increases, the capacities of switches and switch ports that handle the large amount of multimedia information become larger. According to the increase of the capacity of the switch port, the number of sub-ports increases, and thus it is made possible to match multiple subscriber boards to one switch port.
Also, the demand of multicast connection increases as the use of teleconference and Video On Demand (VOD) services increases. Under this network environment, the ATM switching system of the related art has a problem in that the number of connections available for multicasting is limited, because multicast connection IDs corresponding to sub-ports at the side of the egress ports should be reserved in advance for the multicasting connection. Moreover, the sub-port multicast table, which has as many entries as the maximum number of reserved sub-port multicast connections, should be stored and maintained for every egress port, which causes significant hardware overhead in the given system.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.